1. Field of the Invention
This invention relates to new and useful improvements in earth boring apparatus and more particularly to apparatus for directional drilling in the earth.
2. Description of the Prior Art
In directional drilling, maintaining the direction is a major element since most holes are angled and then aimed at a target. In practice, regular surveys must be taken and the bit weight in the stabilized section of the drill string changed as the formations change. The theory of maintaining a straight hole has been well developed. The bit goes where it is pushed or deflected and it is possible to stabilize the drill string to maintain any constant direction and angle.
The main forces acting on a drill bit are the pendulum forces of gravity, leeward forces of the drill string, the wedging effect of formation changes and the walking effect of rotation, especially in tricone bits. These forces may be controlled by proper control of the bit weight, stiffening the drill string by using larger drill collars or square drill collars, and by placement of stabilizers in the collar string.
There are several ways of changing direction in a well. These ways include changing bit weight, changing drill collar stabilization, use of a bent sub, use of a directional whipstock, and jetting. Changing the weight on the bit is one of the most common forms of direction control. Depending upon the arrangement of the stiffening string, the bit can be made to back towards vertical or increase the angle. Changes in stabilizer arrangement can build or decrease angle by using pendulum or lever effects. The bent sub and the whipstock, however, are the tools on which commercial directional drilling has been based. Bent subs and whipstocks, however, require many round trips of the drilling apparatus for only a small amount of hole obtained. As a result, there has been a substantial need for a suitable means for effecting directional control of drilling apparatus, and particularly drilling apparatus utilizing in-hole motors, such as the turbo drill, which does not require repeated trips to the surface of the drilling apparatus.
The use of bit-deflection barrels is known in the prior art, e.g. U.S. Pat. Nos. 3,460,639, 3,298,449 and 3,326,305. The use of deflection barrels in association with in-hole motors is known in U.S. Pat. Nos. 2,637,527 and 3,023,821. U.S. Pat. No. 4,040,495 discloses a bit deflection apparatus for an inhole drilling motor. The prior art referred to above has the disadvantage of either requiring frequent trips of the apparatus or in lacking means to provide a substantial mechanical advantage in application of the bit deflection forces.